Risk, Uncertainty and Priorities - Quantitative Models
445
below the sea bed is not clearly delineated, but original seismic information
leads to the suggestion of a salt top at 300-500 m depth. The lithology under the
salt at location A is not well known!); accordingly we have run computations
with the underlying formations treated as 100% sand, 50% sand and 50% shale,
and 100% shale, respectively.
4.2.1
Burial History and Excess Pressure
Prior to salt insertion, each formation is sequentially deposited and continuous
evolution of porosity, permeability, fluid-flow and overpressure development
computed for prescribed lithologies for each for the formations.
At salt insertion time (determined by taking the "nose" of the salt sheet leftwards at a constant speed until the "nose" intersects location A) the salt is considered to develop as an ever increasing thickness unit inserted at a precise
depth. [For instance if a lateral salt speed of 10 cm/a is taken, with the present
day salt top at 500 m below the present -day seabed, then for a salt "nose" 60 km
to the right of location A, the salt would have been emplaced at location A at 0.6
MaBP. At that time, if the sediment deposition rate above the salt is 100 m/Ma,
then 60 m of the present overburden above salt would need to be removed. Thus
the emplacement depth of the salt would be 440 m (500-60 m) J. In fact, the calculations are a little more complex, because porosity evolution with time and
depth of each formation, including the influence of salt, needs to be included in
the calculation of salt insertion depth.
Two cases were examined: case 1 in which the present-day salt top is at 300 m
below the seabed; case 2 in which the present-day salt top is at 500 m below the
seabed. For each case, the salt thickness was varied, the lateral insertion speed
of the salt was varied, and the lithology underlying the salt was varied from
100% sand through a 50% sand/50% shale mix, to 100% shale. Table 3 provides
the ranges of values used for each of the two cases. Values for each case.
Fig. 12a,b shows, as an example, the predicted excess fluid pressure (in units
ofkgcm- 2 = 1.03 atm) with depth for a salt insertion speed of 10 cm a-I, with the
current salt top at 300 m sub-sea bed, for a 50% sand/50% shale lithology under
the salt and with the salt thickness set at 2000 m (Fig. 12a) and 2200 m (Fig. 12b)
respectively. Fig.12c shows the difference (higher sub-salt excess pressure) from
figure 12a when the lithology is changed to 100% sand and all other parameter
values are held fixed 2 ). For each case similar figures can be drawn, each providing an excess fluid pressure value at breakout just below the salt sheet at the
present -day.
One could, of course, tie seismically to a nearby well if one exists, but one does not know,
as opposed to guess or model, the actual lithologic sequence under the allochthonous
salt sheet
2 Because the salt sheets depicted are of order 100 km in lateral extent, there is no flow of
fluids around the "ends" of the sheets in the short time available. Thus the sand formations are overpressured.
445
below the sea bed is not clearly delineated, but original seismic information
leads to the suggestion of a salt top at 300-500 m depth. The lithology under the
salt at location A is not well known!); accordingly we have run computations
with the underlying formations treated as 100% sand, 50% sand and 50% shale,
and 100% shale, respectively.
4.2.1
Burial History and Excess Pressure
Prior to salt insertion, each formation is sequentially deposited and continuous
evolution of porosity, permeability, fluid-flow and overpressure development
computed for prescribed lithologies for each for the formations.
At salt insertion time (determined by taking the "nose" of the salt sheet leftwards at a constant speed until the "nose" intersects location A) the salt is considered to develop as an ever increasing thickness unit inserted at a precise
depth. [For instance if a lateral salt speed of 10 cm/a is taken, with the present
day salt top at 500 m below the present -day seabed, then for a salt "nose" 60 km
to the right of location A, the salt would have been emplaced at location A at 0.6
MaBP. At that time, if the sediment deposition rate above the salt is 100 m/Ma,
then 60 m of the present overburden above salt would need to be removed. Thus
the emplacement depth of the salt would be 440 m (500-60 m) J. In fact, the calculations are a little more complex, because porosity evolution with time and
depth of each formation, including the influence of salt, needs to be included in
the calculation of salt insertion depth.
Two cases were examined: case 1 in which the present-day salt top is at 300 m
below the seabed; case 2 in which the present-day salt top is at 500 m below the
seabed. For each case, the salt thickness was varied, the lateral insertion speed
of the salt was varied, and the lithology underlying the salt was varied from
100% sand through a 50% sand/50% shale mix, to 100% shale. Table 3 provides
the ranges of values used for each of the two cases. Values for each case.
Fig. 12a,b shows, as an example, the predicted excess fluid pressure (in units
ofkgcm- 2 = 1.03 atm) with depth for a salt insertion speed of 10 cm a-I, with the
current salt top at 300 m sub-sea bed, for a 50% sand/50% shale lithology under
the salt and with the salt thickness set at 2000 m (Fig. 12a) and 2200 m (Fig. 12b)
respectively. Fig.12c shows the difference (higher sub-salt excess pressure) from
figure 12a when the lithology is changed to 100% sand and all other parameter
values are held fixed 2 ). For each case similar figures can be drawn, each providing an excess fluid pressure value at breakout just below the salt sheet at the
present -day.
One could, of course, tie seismically to a nearby well if one exists, but one does not know,
as opposed to guess or model, the actual lithologic sequence under the allochthonous
salt sheet
2 Because the salt sheets depicted are of order 100 km in lateral extent, there is no flow of
fluids around the "ends" of the sheets in the short time available. Thus the sand formations are overpressured.